Williams, who missed the better part of a year with injuries and illness, was back in the semifinals at Flushing for the first time since 2009 when, also on a Saturday night in Arthur Ashe Stadium, she got called for a foot fault against Kim Clijsters, then went on a tirade against the referee that cost her match point.

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"It meant a lot to me to come out here as an American and still be in the tournament," Williams said. "I really wanted to play tomorrow. Such a special day for the United States, so I'm really excited."

And though Wozniacki has the ranking of No. 1 without the benefit of a Grand Slam title, she didn’t let that detail deter her from what could be considered a back-handed compliment.

"I'm still No. 1 in the ranking and still No. 1 in the race," Wozniacki said, according to the AP. "No one can take that away from me right now. Serena played great today. She's a great champion and I wish her well."